Racquet stringers have been used and are well known in the art for tensioning racquet strings in sports racquet such as tennis racquet. Each sport racquet has a different preferred string tension, and even different string compositions may require different string tensions for the same racquet. Lastly player preferences also affect the desired string tension so it is not uncommon to need to tension string as close as possible to a desired tension.
Historically, the racquet stringers were of the simple type having a swing lever and a counterweight mounted movably on the swing lever, in an arrangement commonly known as a drop weight. Examples of this may be found in U.S. Pat. Nos. 3,302,920 and 5,269,515. In these devices, to tighten a string to a specified tension, the operator secures the movable weight on the lever at a specified distance from the fulcrum. The operator then clamps the string on a rotatable wheel coaxial with the fulcrum of the lever and swings the lever against the tension of the string, allowing the pull of gravity on the weight to tension the string in a controlled manner.
However, this type of simple device has two important disadvantages. Firstly, to achieve the correct tension, the swing lever must come to a horizontal resting position against the counter pull of the tension string. Any deviation from the horizontal decreases, in an uncontrolled manner, the tension being applied to the string. Typically it is very difficult to achieve the required tension exactly at a horizontal position of the swing lever. Secondly, the heft of the movable weight and the length of the swing lever create an unwielding implement that needs to be continually manipulated out of the way by the operator during the stringing of a racquet. This promotes operator fatigue as well as creating an obstacle to the free rotation of the racquet being strung. Such free rotation is desirable to allow easy access to the holes on the head of the racquet through which the string must be weaved.
More recently there have been proposed more sophisticated stringers which utilize an electrically driven motor to tension the string. The electrical motor provides the force for the tensioning of strings replacing the drop weight. A variety of configurations have been utilized to tension a string to a specific desired tension. For example, some of the prior art teaches the use of a slipping clutch adapted to slip at a predetermined load as in European patent 00476982. In other cases, the motor is de-energized by the tripping of a switch when the preselected tension is applied to the string simultaneously with the brake being engaged to hold that specific tension (U.S. Pat. No. 3,918,713). Most recently, sophisticated electrical machines including built-in microprocessors have been developed. In one example, the microprocessor compares in each instant the tension on the string to a previously preselected reference tension. Depending upon the results of the comparison, the motor will be engaged or relaxed (U.S. Pat. No. 5,733,212).
Regardless of the approach, electrical machines have, until now, been complex and quite expensive to produce. The reason for this complexity is that the prior art electrical machines have required various and ancillary components such as a clutch to prevent over tensioning the string, a spring loaded element to deflect under tension to cause tripping of a limit switch, or sophisticated electronic circuitry to continually poll sensors and to compare the tension of the string against the preselected tension. The ease of operator use of an electrical machine has therefore come at a substantial price.
Aside from the difficulties associated with the means of tensioning taught by the prior art stringing machines, the actual means of engaging a string as it is tensioned has also had a number of disadvantages. Solid winch elements have had the disadvantage of requiring that the string be wrapped and lapped several times around their perimeter to insure that sufficient friction is provided to prevent the string from slipping as it is pulled. Split pulleys or winches have required that their surfaces be dressed with a rough finish to prevent the string from slipping as it is squeezed between the tightened jaws. The application of this rough finish adds to the cost of production of the winch element and because the finish is applied to a rigid metal or plastic surface, it can cause nicking and damage to the string when squeezing it tightly. Another popular means of engaging the string involves a clamp that reciprocates linearly with the string. The clamp comprises two metal jaws that are drawn together gradually by ball bearings as the string is tensioned. Although this clamp works effective in engaging the string its production costs are comparatively high and far exceeds that of simple circular winch elements.